Methods and Apparatus for Construction of Machine Tools

ABSTRACT

Cement or other liquid-like material fills the hollow tubes of a machine tool under construction. The machine tool structures are held rigidly against a fixture while the substance dries. The machine tool so constructed is relatively lightweight and rigid, and obviates the need for precision machining of large portions of the apparatus.

This application claims the benefit of U.S. Provisional Application No.61/769,740, filed Feb. 26, 2013.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the construction ofmulti-axis machine tools, and methods for the same.

BACKGROUND OF THE INVENTION

A well-known technique for constructing inexpensive linear motion axesutilizes precision ground steel shafts—each fixed at both ends—uponwhich a carriage slides. An example of such a stage, constructed inaccordance with the present invention, is shown in FIG. 3. Crucial tothe proper function of this mechanism are the parallelism of both shaftsand the precise alignment of guide bearings upon which the carriageslides on the precision shafts. Should the distance between the shaftsvary, or if the distance between the shafts does not perfectly matchthat between the guide bearings, binding will occur. Achieving thisprecise alignment typically requires high precision machining ofmultiple different components.

In addition, one of the key challenges facing machine tool designers isto create rigid machine frames which resist tool deflection and whichare also damped sufficiently to suppress vibrations. Another challengeis to create precision alignment both between the bearing elementscomprising each axis, and between the various axes comprising a machine.For example, a standard 3-axis milling machine consists of X, Y, and Zaxes which in the ideal case are perfectly orthogonal to each other.Traditional machine construction techniques rely on bulky castings,forgings, or extrusions to achieve stiffness and damping, and precisionmachining of components to achieve alignment within and between motionaxes. These factors contribute to the cost of fabricating machine tools,and make it difficult to produce machine tools for the mass consumermarket.

SUMMARY OF THE INVENTION

The present invention involves a lightweight (preferably aluminum)extrusion profile capable of providing the basis for most of thecomponents of a linear motion stage without requiring significantprecision post-machining, and a construction technique for utilizingthese elements. Such a stage may be used within an overall frame thatuses inexpensive structural materials and fabrication processes. In thatregard, the invention also includes a construction technique for machinetools which utilizes a precision jig with an aluminum frame filled withcement or epoxy to create precise machine tools, as well as the machinetool so constructed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section for an extrusion template that may be used toform the carriage and end blocks of the preferred embodiment.

FIG. 2 is a stylized perspective view of the end blocks and carriagemade from the extrusion template of FIG. 1 (omitting other machine toolstructures), in their approximate orientation with one another.

FIG. 3 is a perspective view of a complete linear motion stage inaccordance with the preferred embodiment.

FIG. 4 is a perspective view of several linear motion stages of FIG. 3,stacked and oriented orthogonally.

FIG. 5 is a detail showing epoxy between a floating bushing and thecarriage of the preferred embodiment.

FIG. 6 is a stylized perspective view of two orthogonal linear motionstages exploded with respect to one another, showing the T-slots on thework surface of the vertical stage.

FIG. 7 is an alternative embodiment to the template of FIG. 1.

FIG. 8 is the template of FIG. 7 showing, in cross section, where afloating bushing would be glued in with epoxy and surrounded on twosides.

FIG. 9 is a perspective view of a machine tool constructed in accordancewith the preferred embodiment.

FIG. 10 is a perspective view of one length of thin-walled aluminum boxextrusion.

FIG. 11 is a detail of the extrusion of FIG. 10, showing a press fitinsertion of a screw hole.

FIG. 12 is a perspective view of aspects of the machine tool of FIG. 9,during the process of construction.

FIG. 13 is a cross section of a portion of the machine tool of FIG. 9.

FIG. 14 is a perspective view of a jig used to construct the machinetool of FIG. 9.

FIG. 15 is another cross section of a portion of the machine tool ofFIG. 9.

FIG. 16 is a detail of the clamping structures used in the manufacturingprocess of the embodiment of FIG. 9.

FIG. 17 is a perspective view of the clamping structures and jig used inthe manufacturing process of the embodiment of FIG. 9.

FIG. 18 is a detail showing taping of the box extrusion of FIG. 10during the fill process.

FIG. 19 is a perspective view of a partially completed manufacture ofthe embodiment of FIG. 9.

FIG. 20 is a detail of the shaft structure of the embodiment of FIG. 9.

FIG. 21 is another detail showing the box extrusion of FIG. 10 duringthe fill process.

FIG. 22 is a detail of the extrusion of FIG. 10, showing electricalwiring passing through a hole.

DETAILED DESCRIPTION OF INVENTION:

A complete linear motion stage constructed in accordance with thisinvention is shown in FIG. 3. Two guide shafts 301 and 302 areconstrained at each end in two end blocks 303 and 304. The purpose ofthese end blocks is to support the guide shafts and to constrain themparallel to each other. Both end blocks are rigidly anchored to a baseplate 305. A carriage rides 306 on bearings fixed 307 to the carriage306, and these bearings slide on the guide shafts. In the embodiment ofFIG. 3, a leadscrew 308 driven by a stepper motor 309 provides themotive force and position control of the carriage relative to the endblocks. However other drive mechanisms such as pulleys and belts, racksand pinions, etc. could be used. FIG. 2 demonstrates how a singleextrusion profile (e.g., of FIG. 1) can be used as the basis for endblocks and carriage, while providing supplementary functionality such asmotor mounting features, and t-slots which permit the extrusion tofunction as a machine work surface.

Preferably, the guide shafts 301 and 302 remain parallel along theirentire length, so that the carriage moves smoothly. In order toaccomplish this, the features in both end blocks which anchor the guideshafts are identically spaced. This is accomplished by two guide shaftseat features 101 and 103 in the extrusion profile (FIG. 1), one at eachend of the extrusion profile. Because both end blocks are cut from thesame extrusion, the distance between both features remains constant onboth blocks. However it is possible that during the extrusion process,variation in the center distance between the shaft seat features mayoccur along the length of the extrusion or between lots of extrusion. Ifthe variation proves to be within acceptable limits, the shafts can bepressed directly into the seats or the seats could be slightlyundersized and then reamed to the shaft diameter. If the variation inseat center distance is too great, the seats can be manufacturedundersized and then machined to size and in the proper locations on aCNC machining center with minimal material removal.

Preferably, the carriage slides smoothly and with minimal wear on theguide shafts. Rather than manufacture the carriage from a suitablebearing material with the necessary surface finish, a common techniqueis to attach off-the-shelf bearings to the carriage. In the presentembodiment two bearings (i.e. a bearing “pair”) are used per shaft.Preferably, the center distance between the bearing pairs is identicalto that between the shafts if the carriage motion is to be smooth andlow-friction. In order to accomplish this, one pair of bearings ispress-fit into the carriage, while the other pair is left floating whilesurrounding the shaft, and then glued in-place onto the carriage. Thisconstruction technique permits the distance between the guide shafts tobe copied to the bearings without requiring that the distance betweenthe guide shafts is known during the manufacture of the carriage.

The two fixed bearings are pressed into the fixed bearing seat feature101 (FIG. 1), which is formed by reaming the guide shaft seat feature toa size suitable for a press fit on the bearings. Because both bearingsin the set are press-fit into a common hole (typically one from eachside), they are de facto collinear. During the pressing process it mayalso help to install a shaft in both bearings to ensure that they pressin straight.

The two floating bearings are glued in place once the stage is mostlyassembled. One of the two guide shafts is slipped through the fixedbearing sets, and the floating bearings are slipped onto the othershaft. Both shafts are then either pressed or slipped into the guideshaft seat features in the end blocks. If a slip-fit is used, additionalreinforcement such as adhesive, set screws, or other means can be usedto fix the guide shafts to the end blocks. Once the stage has been thusassembled, the distance between the guide shafts is set and the floatingbearings can be glued to the carriage. The floating bearing seat surface103 (FIG. 1) is designed so that when the bearings rest on this surface,the top surface of the carriage is parallel to the top surfaces of theend blocks 304 and 303 when the carriage 306 is oriented relative to theend blocks according to FIG. 3. With the bearings that enclose shaft 302firmly resting on the floating bearing seat surface 103, adhesive isapplied to the gaps between the bearings and the floating bearing seatsurface, and the glue surface (FIG. 1). FIG. 4 shows an assembledcarriage 306 from below, where epoxy was used as the adhesive betweenthe floating bearing and the bearing seat surface 103. FIG. 5 is adetail showing epoxy applied between the bearing and the seat surfaces.The glue between the bearing and the glue surface acts in shear, greatlyincreasing the strength of the joint. FIGS. 7 and 8 illustrate 701 and801 an improved design in which glue surfaces are located on both sidesof the bearing, which increases resistance to peeling.

FIG. 4 shows how several axes constructed in this manner can be stackedto form multi-axis motion stages. FIG. 6 further elaborates on thispoint, and also shows how the same extrusion can be used to create thesurface form of a work surface (the vertical rectangle as depicted inthe figure) by including t-slot features into the extrusion.

Additional features are provided in the extrusion profile (FIG. 1) suchas t-slots, motor mounting holes, and a hole which both clears the leadscrew when used as an end block, and provides a mounting feature for alead nut when the extrusion profile is used as a carriage.

Turning to a second aspect of the present invention, a method ofinexpensively manufacturing a machine tool is described. FIG. 9 shows athree-axis milling machine constructed in accordance with the invention.The structural frame of this machine is fabricated from thin-walledaluminum box extrusion 901 which has been filled with cement. Howeverother fill materials such as epoxy for example may be used. FIG. 10shows two such thin-walled box extrusions which have had features milledin them such as:

-   -   miter cuts at various locations to permit folding 1001 and 1002    -   holes which allow various elements such as motor standoffs or        precision rods to pass into the interior of the tube, 1004    -   holes which pass entirely through the tube for elements such as        lead screws 1003    -   fill ports into which cement will be poured, or which allow        cement to flow from one tube to another.1005 and 1006    -   holes 1101 into which threaded inserts can be pressed (as in        FIG. 11).

FIG. 12 illustrates these fabricated extrusions 1201 bent and installedon a precision jig 1202. For the machine of FIG. 9, the structural frameis comprised of one extrusion that has been bent into a rectangularframe for the Y axis 902 and another extrusion bent into an upside-downU 903 for the X axis. The purpose of the jig is to properly align andconstrain the elements of the machine during the casting process. Thejig is not part of the machine tool itself.

The cross-section in FIG. 13 shows several elements of the structure.Most notable is the use of threaded rod 1301 and 1302 passing betweenthe X and Y axis tubes whose purpose is to reinforce the cementinterface between these two elements. Additionally, a cardboard 1303pass-through tube is used to create a channel through the tube for thelead screw, and motor standoffs can be seen passing through one wall ofthe X axis tube.

FIG. 14 is a drawing of the precision jig, which consists of threeparts: a base plate 1401 that also acts to fixture the Y axis tube andmotion components, a vertical back plate 1402 mounted perpendicular tothe base plate, and a sliding X axis fixture 1403 used to fixture the Xaxis tube and motion components. The guide shafts 1501, 1502, 1503 and1504 of each axis rest in shaft alignment features as shown in FIG. 15.Additionally, alignment pins are used to accurately locate the leadscrew pass-through holes on the X and Y axes relative to the fixture.Clamps 1601 shown in FIG. 16 preload the shafts into the jig's alignmentfeatures during casting. FIG. 17 shows the structural tubes 1701, 1702,1703 being clamped in place on the fixture, and motion componentsinstalled and fixtured to the jig. Tape is used to seal gaps in thestructural tubes to prevent cement from leaking (detail shown in FIG.18). The final product post-casting is shown in FIG. 19.

The precision of the resulting machine is derived from the precision ofthe casting jig, not from the fabrication tolerances of the structuraltubes. When the tubes are flooded with cement, the alignment of the jigis permanently copied to the machine. FIG. 20 illustrates thesignificant gap between one of the shaft entry holes 2001 and the guideshaft 2002 itself. This shaft entry hole could have been mis-located byseveral millimeters without affecting the final position of the shaft.

During the casting process, cement may be poured from the top of themachine through fill ports 2101—one of which is shown in FIG. 21. If thefill material is sufficiently non-viscous, the pressure generated bypouring from the top of the machine helps to fill oddly-shaped voids inthe structure. Vibrating the entire machine and fixture can also helpprevent air bubbles. The material preferred for constructing in themachine of FIG. 9—gypsum cement—has the property that it expands whilecuring. It is intended that this expansion, limited locally by thetubular structure, places the cement under compression. This isbeneficial because cracks are less likely to occur in materials whichare under compression. Expansion agents may be chosen along with thefiller material to further increase this effect.

FIG. 22 illustrates the routing of electrical and other cables 2201.Because the structure is comprised of tubes, it is possible to reducecable routing clutter by running the cable through the tubes prior tothe casting process.

In conclusion, the use of thin-walled tubular extrusion filled with acast-able material on a precision jig offers several benefits overtraditional machine construction techniques.

-   -   High stiffness and damping is achieved by the low-cost cast        material in conjuncture with the tube material.    -   High precision fabrication is only done once during the creation        of the jig, after which many high-precision machines can be        manufactured from low-precision fabricated tubes.

While the invention has been described with particular reference tospecific embodiments, it will be apparent to those skilled in the artthat the same principles may be used in similar arrangements. Theinvention is not limited to the precise structures described. Variouschanges and modifications may be made without departing from the spiritand scope of the invention as defined by the claims below. For example,steps of a process can be in any order, unless the context clearlyindicates otherwise.

What is claimed is:
 1. Apparatus comprising, in combination: (a) a firstlinear stage that comprises a first set of shafts and a first carriage,the first set of shafts being configured to guide linear motion of thefirst carriage; (b) a second linear stage that comprises a second set ofshafts and a second carriage, the second set of shafts being configuredto guide linear motion of the second carriage; and (c) one or moretubes; wherein one or more spaces inside the one or more tubes arefilled with cement.
 2. The apparatus of claim 1, wherein the apparatusis configured to control position of a milling device that is attachedto the apparatus.
 3. A method of fabricating a positioning tool, whichmethod comprises: (a) changing the shape of one or more tubes while (i)the one or more tubes are hollow, and (ii) the one or more tubes areeither pressed against a fixture or constrained from moving in at leastone direction by the fixture; (b) moving a filler into the one or moretubes while the filler is in a liquid state, which filler solidifieswhile the one or more tubes are either pressed against the fixture orconstrained from moving in at least one direction by the fixture; and(c) aligning at least one shaft in a first set of elongated shafts to beparallel with a first axis, and (d) aligning at least one shaft in asecond set of elongated shafts to be parallel with a second axis, thesecond axis being perpendicular to the first axis; wherein, whenfabrication of the positioning tool is complete (i) the one or moretubes are structural elements of the positioning tool, (ii) the firstset of shafts is configured to guide linear movement of a firstcarriage, (iii) the second set of shafts is configured to guide linearmovement of a second carriage, and (iv) the first and second sets ofshafts and the first and second carriages are parts of the positioningtool.
 4. The method of claim 3, wherein, while the filler solidifies,each shaft, out of the first set of shafts, is either pressed againstthe fixture or constrained from moving in at least one direction by thefixture.
 5. The method of claim 4, wherein, while the filler solidifies,each shaft, out of the second set of shafts, is either pressed againstthe fixture or constrained from moving in at least one direction by thefixture.
 6. The method of claim 3, wherein the method further comprisesattaching a milling device to the positioning tool.
 7. The method ofclaim 3, wherein the one or more tubes comprise metal.
 8. The method ofclaim 3, wherein the filler comprises cement.
 9. The method of claim 8,wherein the cement comprises an expansive cement or includes expansionagents.
 10. The method of claim 3, wherein the method further comprisespositioning rebar inside the one or more tubes, before the fillersolidifies.
 11. The method of claim 3, wherein the filler comprises apolymer.
 12. The method of claim 3, wherein: (a) the aligning in claim(3)(c) includes positioning a specified shaft, out of the first set ofshafts, such that (i) a first portion of the specified shaft extendsinto or through a hole in a first block, (ii) a second portion of thespecified shaft extends into or through a hole in a second block, and(iii) a third portion of the specified shaft extends through a hole inthe first carriage; and (b) the first block, second block and firstcarriage are three separate parts that were cut, before the positioning,from a single integral extrusion, which extrusion did not include theone or more tubes.
 13. The method of claim 3, wherein the aligning inclaim (3)(c) includes positioning a certain shaft, out of the first setof shafts, such that (i) a first portion of the certain shaft extendsinto a hole in a first region of the one or more tubes, and (ii) asecond portion of the certain shaft extends into or through a hole in asecond region of the one or more tubes.
 14. The method of claim 3,wherein: (a) the first set of shafts includes a first shaft and a secondshaft; and (b) the aligning in claim 3(c) includes (i) positioning thefirst shaft such that the first shaft extends through a first hole inthe first carriage, and (ii) positioning the second shaft such that thesecond shaft extends through a second hole in a bushing; and (iii) whilethe first shaft extends through the first hole and the second shaftextends through the second hole (A) causing the bushing to undergo atranslation from a first position relative to the first carriage to asecond position relative to the first carriage, such that the distancebetween the first and second shafts changes, and (B) after thetranslation, forming a rigid attachment between the bushing and thefirst carriage, such that, when the rigid attachment is formed, thebushing is rigidly positioned in the second position.
 15. The method ofclaim 3, wherein: (a) a joint links a first part and a second part, thefirst part being included in the fixture and the second part beingincluded in the fixture or in the second linear stage; (b) the joint isadjustable, such that adjustment of the joint causes the first andsecond parts to translate relative to each other; (c) the first carriageincludes a planar surface; (d) a first plane is in-plane with the firstplanar surface; (e) the method further comprises using the joint toadjust the minimum distance between the second set of shafts and thefirst plane, while the joint prevents rotation of the second set ofshafts relative to a point in the first plane.
 16. The method of claim3, wherein: (a) the method further comprises aligning at least oneshaft, out of a third set of shafts, to be parallel with a third axis,the third axis being perpendicular to both the first and second axes;and (b) when fabrication of the positioning tool is complete (i) thethird set of shafts are configured to guide linear motion of a thirdcarriage, and (ii) the third set of shafts and the third carriage areparts of the positioning tool.
 17. The method of claim 3, wherein,before the filler is moved into the one or more tubes, at least oneelectrical wire is positioned to extend into a hollow interior region ofthe one or more tubes.
 18. The method of claim 3, wherein changing theshape in claim 3(a) includes bending the one or more tubes, aftermitered cuts have been made in the one or more tubes.
 19. A method offabricating a positioning tool, which method comprises: (a) changing theshape of one or more tubes while (i) the one or more tubes are hollow,and (ii) the one or more tubes are either pressed against a fixture orconstrained from moving in at least one direction by the fixture; and(b) moving a filler into the one or more tubes while the filler is in aliquid state, which filler solidifies while the one or more tubes areeither pressed against the fixture or constrained from moving in atleast one direction by the fixture; and wherein (A) a first block, asecond block and a first carriage are three separate parts that werecut, before the positioning, from a single integral extrusion, whichextrusion did not include the one or more tubes; (B) the method furthercomprises positioning a specified shaft, out of the first set of shafts,such that (i) a first portion of the specified shaft extends into orthrough a hole in a first block, (ii) a second portion of the specifiedshaft extends into or through a hole in a second block, and (iii) athird portion of the specified shaft extends through a hole in the firstcarriage; and (C) when fabrication of the positioning tool is complete(i) the one or more tubes are structural elements of the positioningtool, (ii) the first set of shafts is configured to guide linearmovement of a first carriage, and (iii) the first set of shafts and thefirst carriage are parts of the positioning tool.
 20. The method ofclaim 19, wherein the filler comprises cement.